[llvm] [MC] Remove a redundant declaration of createNullStreamer (NFC) (PR #166025)

[Shilei Tian via llvm-commits]

https://github.com/shiltian commented:

Well, I think this is just a forward declaration such that the header doesn't need to be included. Not sure how this will help.

https://github.com/llvm/llvm-project/pull/166025